Introduce a setting to show net prices (#415)

* Introduce a setting to show net prices in the frontend

* Show net prices in the backend as well

src/pretix/base/models/items.py

@@ -11,6 +11,7 @@ from django.utils.functional import cached_property
 from django.utils.timezone import now
 from django.utils.translation import ugettext_lazy as _
 
+from pretix.base.decimal import round_decimal
 from pretix.base.i18n import I18nCharField, I18nTextField
 from pretix.base.models.base import LoggedModel
 
@@ -221,6 +222,11 @@ class Item(LoggedModel):
         if self.event:
             self.event.get_cache().clear()
 
+    @property
+    def default_price_net(self):
+        tax_value = round_decimal(self.default_price * (1 - 100 / (100 + self.tax_rate)))
+        return self.default_price - tax_value
+
     def is_available(self, now_dt: datetime=None) -> bool:
         """
         Returns whether this item is available according to its ``active`` flag
@@ -313,6 +319,11 @@ class ItemVariation(models.Model):
     def price(self):
         return self.default_price if self.default_price is not None else self.item.default_price
 
+    @property
+    def net_price(self):
+        tax_value = round_decimal(self.price * (1 - 100 / (100 + self.item.tax_rate)))
+        return self.price - tax_value
+
     def delete(self, *args, **kwargs):
         super().delete(*args, **kwargs)
         if self.item:

src/pretix/base/models/orders.py

@@ -7,10 +7,11 @@ from typing import List, Union
 
 from django.conf import settings
 from django.db import models
-from django.db.models import F
+from django.db.models import F, Sum
 from django.db.models.signals import post_delete
 from django.dispatch import receiver
 from django.utils.crypto import get_random_string
+from django.utils.functional import cached_property
 from django.utils.timezone import now
 from django.utils.translation import ugettext_lazy as _
 
@@ -215,6 +216,18 @@ class Order(LoggedModel):
         else:
             self.payment_fee_tax_value = Decimal('0.00')
 
+    @property
+    def payment_fee_net(self):
+        return self.payment_fee - self.payment_fee_tax_value
+
+    @cached_property
+    def tax_total(self):
+        return (self.positions.aggregate(s=Sum('tax_value'))['s'] or 0) + self.payment_fee_tax_value
+
+    @property
+    def net_total(self):
+        return self.total - self.tax_total
+
     @staticmethod
     def normalize_code(code):
         tr = str.maketrans({
@@ -432,6 +445,10 @@ class AbstractPosition(models.Model):
             else:
                 q.answer = ""
 
+    @property
+    def net_price(self):
+        return self.price - self.tax_value
+
 
 class OrderPosition(AbstractPosition):
     """